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  • Author or Editor: W. L. Physick x
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W. L. Physick and N. J. Tapper

Abstract

Observations of surface winds in the vicinity of a moderate-sized (about 70 km2) dry salt lake by Tapper suggest that differences in albedo and soil thermal properties between the salt and surrounding sand surfaces may be strong enough to drive a mesoscale thermal circulation. In this paper a numerical mesoscale model is used to investigate disturbances generated by moderate and large-sized (about 7000 km2) lakes. In the latter case, a typical size for the great salt lakes of inland Australia, disturbances with strong horizontal and vertical shear are found at a distance of more than 200 km from the lakeshore by midnight. The separate contribution of albedo and soil property differences to the circulation are assessed, under both dry and moist soil conditions. The possible influence of such circulations on regional climate is discussed and it is suggested that salt lakes may be a source for solitary waves observed in the Australian interior.

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D. J. Thomson, W. L. Physick, and R. H. Maryon

Abstract

The problem of how to formulate random walk dispersion models in situations where the flow properties vary discontinuously across an interface is considered. It is shown how the dispersion model can be made consistent with the assumptions made about the turbulence. The approach does not lead to a unique model, but it is argued that in many cases the rate of diffusion through the interface is limited not by the detailed physics of the interface but by the rates of diffusion on either side of the interface and, in such situations, results may be insensitive to which of the consistent models is chosen. Some simulations are presented to illustrate these ideas.

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W. L. Physick, W. K. Downey, A. J. Troup, B. F. Ryan, and P. J. Meighen

Abstract

Phase I of the Cold Fronts Research Programme was carried out during November/December 1980 in south-eastern Australia. Data from a frontal event on 27 November are analyzed, with particular emphasis on a squall line that formed over the Southern Ocean and moved across the predominantly land-based network.

Potential instability between 750 and 550 mb, arising from the subsynoptic scale circulation associated with the frontal system, existed ahead of the squall fine. Boundary layer forcing resulting from convergence of the cold outflow and presquall air was responsible for release of this instability as the line passed through the observing network.

The lowest 200 m of the nocturnal boundary layer were undisturbed by the cold outflow, as this radiatively cooled layer was potentially colder than downdraft air. A surface pressure jump of 3 mb was recorded and the low-level wind rotated through 360° in two hours in response to the associated mesohigh.

Two-dimensional (xz) cross sections of wind and thermodynamic variables composited from aircraft, radiosonde and pibal data reveal centre of upward motion at 800 mb near the leading edge of the outflow and 50 km behind at 500 mb, with the latter being the stronger of the two. Cooling extends to 620 mb and the important role played by relatively dry air entering the system from the rear at middle levels is clearly shown.

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M. Segal, W. L. Physick, J. E. Heim, and R. W. Arritt

Abstract

The thermal impact of differential cloud shading across a cold front is evaluated briefly through conceptual, scaling, and numerical-modeling approaches. It is suggested that in summer the shading may enhance the boundary-layer average thermal contrast across the front by as much as 5 K for prolonged shading over the cold sector and with a dry surface in the warm sector. For short shading duration or wet surfaces along the warm sector, the thermal impact of shading reduces significantly. It is concluded that the shading effect may provide a pronounced contribution to frontogenesis for weak or moderate cold fronts.

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J. R. Garratt, W. L. Physick, R. K. Smith, and A. J. Troup

Abstract

Observations of four cold-frontal systems traversing the coastal region of southeast Australia in late spring and early summer are described in terms of process occurring on the mesoscale. A conceptual model is presented which summarizes the main results of the data analysis. Features found in common with other studies of cold fronts include:

(i) the multiple-line nature of the frontal transition zone (FTZ);

(ii) concentration of cyclonic relative vorticity at a height z≈1 to 1.5 km in the rear of the FTZ; and

(iii) the existence of a prefrontal jet at z≈1.5 km, northerly in our case, southerly in the Northern Hemisphere.

The change lines within the FTZ (and at the leading edge if there is no sea breeze) are most probably convective instability lines whose alignment and movement depend on the large-scale, cloud-layer winds. The lines are evident as mesoscale cloud bands from satellite imagery and as rainbands from radar. At least one of these develops into a vigorous squall line whose cold outflow produces a pressure jump, and related wind-shift line. Movement of the pressure-jump line depends both on the gravity-current nature of the cold outflow and the environmental wind field. The squall line and pressure-jump line are associated with mesoscale high and low pressure features to which the boundary-layer wind field responds.

The structure of the FTZ up to z=2 km appears to be dominated by the presence of the squall line, with upwards motion ahead and downwards behind. On a horizontal scale of 100 km, cyclonic vorticity reaches twice the Coriolis parameter f in the vicinity of the squall line. Frontogenesis occurs largely within the FTZ with horizontal convergence and deformation processes being of comparable importance.

The prefrontal jet is broadly in thermal wind balance with the horizontal temperature gradient which is, itself, determined by the fact that prefrontal air closest to the FTZ originates farther to the north and is therefore hotter than prefrontal air more distant from the zone.

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